The interaction of thrombin with platelets will be analyzed in an attempt to answer specific questions. The previosuly described transition from a rapidly to a more slowly dissociable state will be investigated in an attempt to ascertain its possible physiological significance. Experiments will also be carried out to attempt to identify that component of binding that is responsible for platelet activation. Finally, photoachivatable cross-linking of thrombin to platelets will be refined to attempt a better identification of major binding proteins.